Field of the Invention
The present invention relates to optical interferometers, and, more particularly, to interferometric sensors such as fiber optic gyroscopes.
Description of the Related Art
Fiber optic rotation sensors typically include a loop of fiber optic material to which light waves are coupled such that a pair of light waves propagate around the loop in opposite directions (i.e., the two light waves are counterpropagating). When the loop is rotated, a relative phase difference is induced between the counterpropagating light waves in accordance with the well-known "Sagnac effect." The amount of phase difference corresponds to the rotational velocity of the loop. The counterpropagating light waves, when recombined, interfere constructively or destructively to produce an optical output signal that varies in intensity in accordance with the rotation rate of the loop. Rotation sensing is commonly accomplished by detection of this optical output signal.
A number of devices and techniques have been developed to process the detected optical output signal to provide an electrical output signal that represents the velocity and direction of rotation of the loop. Known devices and techniques monitor the intensity of the optical output signal to measure the phase difference between the counterpropagating light waves to determine the rotational velocity and direction in accordance with the Sagnac equation: ##EQU1## where: .DELTA..PHI..sub.R is the Sagnac phase difference caused by rotation of the optical loop;
A is the area bounded by the optical loop in which the light waves counterpropagate; PA0 N is the number of times that the light waves propagate around the loop before being recombined; PA0 .lambda. and c are the free space values of the wavelength and velocity, respectively, of the light waves applied to the optical loop.
q is the angular velocity of the loop about an axis that is perpendicular to the plane of the loop; and
The intensity of the optical output signal is a function of the Sagnac phase difference .DELTA..PHI..sub.R between the two counterpropagating light waves as follows: EQU I.sub.T =I.sub.1 +I.sub.2 +2.sqroot.I.sub.1 I.sub.2 cos (.DELTA..PHI..sub.R) (2)
where I.sub.T is the intensity of the optical output signal, and I.sub.1 and I.sub.2 are the individual intensities of the two counterpropagating light waves.
It has been found that simple measurement of the intensity of the optical output signal will not provide sufficient information from which the direction and rate of rotation can be determined. For example, the sensitivity of the phase difference measurement is effectively zero for phase differences that are integral multiples of .pi. (i.e., .DELTA..PHI..sub.R =N.pi. for N=. . . -2,-1,0,1,2, . . . ) because the interference intensity is an even, periodic function of the phase difference (i.e., the interference intensity varies as a cosine function as set forth in Equation 2 above. Thus, small phase differences cannot be directly measured near a zero rotation rate. Typically, this difficulty in measuring small phase differences is overcome by dynamic biasing wherein an additional phase modulation is introduced into the closed path around which the light waves are propagating. A time-varying modulation with a zero mean amplitude is generally used rather than a static modulation. The dynamic biasing causes the phase signal to reach values where the sensitivity is suitably large so that the phase difference is readily measurable.
A gyroscope with dynamic biasing can be readily used to measure small rotation rates. When the phase modulation is a harmonic signal of a predetermined frequency, a small phase shift caused by the rotation of the optical loop causes the intensity of the optical output signal to include a time-varying component at the predetermined frequency. This time-varying component can be demodulated to provide a measure of the Sagnac phase shift and thus the rotation rate. Although the dynamic biasing allows phase differences to be measured with high sensitivity at low rotation rates, the rotation-induced interference intensity is periodic and thus cannot be used directly to measure high rotation rates.
In order to provide an extended dynamic range for measuring rotation rates, two basic approaches have been used. One approach is to provide a feedback signal from the demodulated optical output signal to a second device in the optical loop. The second device provides an additional, controllable, non-reciprocal phase difference and the feedback signal is varied so as to null the demodulated optical output signal. The feedback signal required to null the demodulated optical output signal is monitored to measure the Sagnac phase shift and thus the rotation rate. The performance of this type of closed-loop gyroscope depends in part upon the stability, linearity and phase range of the non-reciprocal phase shifter used for the second device and upon the characteristics of the electronics that provides the feedback signal.
An alternative approach to extending the dynamic range of an optical fiber gyroscope is an open-loop approach wherein no feedback is provided to null the demodulated output signal. All the information necessary to reconstruct the Sagnac phase difference is included in the interference intensity caused by the combination of the rotation and the dynamic modulation. Typically, extensive signal processing is required to extract the phase difference information from the optical output signal when the phase difference is not limited to values near zero. When an extended dynamic range is to be attained, the signal processing can be quite demanding. In many designs of open-loop optical gyroscopes and other interferometers, the quality of the electronic signal processing circuitry, rather than the quality of the optical circuit, determines the dynamic range and accuracy of the sensing system.
Both analog and digital processing may be used to process the optical output signal, and, it is preferable that most of the processing be performed in the digital domain. However, typical commercially available analog-to-digital convertors needed to convert the analog electrical representation of the optical output signal to digital data do not have sufficient dynamic range to provide quality digital processing. Thus, complex analog circuitry is typically required as part of the electronics that processes the optical output signal. Other electronic circuitry has been used that relies less heavily upon digital processing. However, generally, the electronic components must be carefully selected and stabilized in order to achieve the high accuracy that is required to provide a sensitive interferometer with an extended dynamic range.